1. Field of the Invention
The invention relates to a thermal transfer printing device for a franking and/or addressing machine.
In the thermal transfer printing process, a printing medium is usually led between a thermal printing head and a backing roller disposed opposite the latter, on the side of the backing roller. An ink ribbon is led through on the side of the thermal printing head. It is necessary for the printing head to rest on the ink ribbon with an appropriately high force and for the ribbon to in turn rest on the printing medium, in order to transfer molten ink particles from the ink ribbon to the printing medium.
The ink may be bonded to the ink ribbon in a wax layer, or recently in a layer of polyimide as well.
In the case of an office printer which works on the thermal transfer printing principle, operations are carried out with constant printing relationships, such as the same contact force for the thermal printing head, single sheets of a prescribed paper grade, in particular paper thickness, and constant paper guidance.
Contrary to the office printer, in the case of franking and/or addressing machines, the finish of the letters, in particular their thickness and stiffness, frequently changes because of different fillings and the paper quality of the envelopes.
The printing relationships become particularly critical in the case of processing air mail, since the envelopes used therefor are often very soft and sensitive. Although more careful operation would be possible by reducing the contact pressure, the print quality would then in turn be correspondingly impaired. On the contrary, in the case of so-called soft letters, the contact pressure must even be increased with respect to normal letters in order to achieve the same print quality. However, higher contact forces in turn result in greater wear on the thermal printing head and, accordingly, to a correspondingly shortened service life, and may additionally lead to damage to the letters.
Published European Patent Application 0 787 592 A1 discloses a thermal printer in which the letters are led along while lying flat along the broad side of a thermal printing head. The heating elements are disposed in the center of the plate-like printing head, in a row transverse to the transport direction of the letter or envelope. The letter, together with the ink ribbon, is pressed against the heating elements through the use of a spring-mounted roll. The broad side is significantly larger in comparison with the region having the heating elements, so that the ink ribbon and letter, following the printing operation, are guided in such a way that they rest on one another for a relatively long time.
Due to the contact over a large area, that method of guiding the letters certainly takes care of the paper, but the ink particles are more difficult to detach from the ink ribbon. Compensation is possible only through the use of a longer thermal action time, which in turn necessitates a lower transport speed. In addition, because of the greater contact area, a higher contact force is required, as a result of which once more the thermal printing head wears more rapidly.
The conditions are similar in another franking machine known from Published European Patent Application 0 724 234 A2, in which the row of heating elements is displaced more to one end of the plate-like thermal printing head. However, the joint path of the ink ribbon and the letter, following the thermal action, is still too long, since the ink ribbon is only led away from the letter by a deflection roller disposed downstream.
Furthermore, a thermal transfer printing device is known from European Patent 0 434 340 B1, in which the heating elements are fitted to one edge of a thermal printing head, and the latter only presses in the direction of a backing roller with that edge. The ink ribbon is pivoted away from the letter over a roll disposed downstream of the edge. In that case too, the residence time of the ink ribbon on the letter is still too long. Added to that is the fact that the edge of the thermal printing head, which is provided with a small radius of curvature, loads the letter mechanically so severely that, in the case of envelopes made of thin paper and having a multilayer soft filling, it is possible for creases to arise, extending through to damage. In addition, that results in a correspondingly poorer print quality.
Finally, a thermal printer is further known from Published European Patent Application 0 329 478 A1, in which the heating elements are fitted directly to a narrow front edge of a flat thermal printing head, and the ink ribbon is led away from the printing medium directly downstream of the heating element printing bar. Favorable conditions are provided in that way in relation to the detachment of ink particles, but any use for multilayer printing media, such as soft letters, is associated with the risk of creasing.
That thermal printing head represents the so-called "corner-edge type", while the two devices described at the beginning correspond to the so-called "flat type".